The present invention relates to the field of absorbent papers based on cellulose wadding, for sanitary or domestic use such as bathroom tissue, paper towels or other wiping paper, paper napkins, etc.
To produce such products, cellulose wadding also called tissue paper is usually used. It is an absorbent paper of low basis weight, lying between 10 and 45 g/m2, obtained by the wet method from paper fibers. It comprises, where appropriate, chemical additives in small proportions, depending on the use for which it is intended. It may be obtained by pressing the still-wet sheet on a large-diameter, heated cylindrical element, on which it is dried and from which it is subsequently detached by means of a metal blade applied against the latter, across its direction of rotation. The purpose of this operation is to crepe the sheet which then has undulations across its direction of travel. The creping confers a certain elasticity on the sheet at the same time as it increases the thickness thereof and gives it touch properties.
Another known manufacturing method comprises a first step of drying the sheet, at least partly, by means of a current of hot air passing through it. The latter may or may not then be creped.
Usually, the sheet thus manufactured is then transformed in another distinct manufacturing phase, called transformation or converting, and combined with other sheets then called plies to form the end product of absorbent paper.
Specifically, when the requirement is to confer particular properties on a sheet such as thickness, softness, bulk, it is possible to choose to combine several plies together.
The combining operation may be of a chemical nature by adhesive bonding for example or else of a mechanical nature.
Concerning adhesive bonding, the known methods consist in depositing a film of adhesive over some or all of the surface of one of the plies, then placing the adhesive-treated surface in contact with the surface of at least one other ply.
This type of combining operation requires specific additional equipment in the production line which represents a cost and added technical difficulties. In addition, the adhesive is expensive in itself, soils the cylindrical elements of the embossing unit and may cause an added rigidity that is undesirable on the end product whose softness will be further diminished by the presence of the adhesive. These disadvantages have caused certain manufacturers to turn towards mechanical-type combining operations.
In this case, the plies may be combined by knurling or by compression in a transformation or converting phase.
Knurling consists in compressing the plies to be combined between a knurling wheel (or engraved wheel provided with elements in relief) and a smooth cylindrical element.
Each knurled strip therefore corresponds to the width of a knurling wheel. The strips may form decorative strips on the sheet.
As an illustration, U.S. Pat. No. 3,377,224 describes a tissue paper made by such a method. Given that a very limited width of paper is knurled, a notable disadvantage lies in the delamination of the zones that are not knurled.
In addition, combining by knurling is limited when it is required to produce patterns over the whole width. Specifically, even if a large number of knurling wheels are placed side by side (thus creating a large number of strips), there may still remain zones that are not knurled.
Document EP 1 362 953 illustrates a particular example of an installation and a method using knurling. The major difference compared with the basic method described hereinabove lies in that the plies are combined along wide parallel strips (direction of travel of the machine) on the sheet, and in that a film of additive such as oil is applied to at least one of the faces of the sheet.
Furthermore, knurling generally creates problems of visibility of the embossed pattern if there is one, because the knurling flattens the embossed patterns.
In addition, in the case where a large number of knurling wheels is used, the adjustment and/or setting of the knurling wheels makes manufacture difficult and complex.
Embossing is also known that is a deformation in the thickness of the sheet or of the ply, which confers thereon a particular relief or indentation. The thickness of the sheet or of the ply is increased after embossing compared with its initial thickness.
Although embossing adds a thickness to each ply or sheet, it nevertheless induces a substantial reduction in the sheet's resistance to tearing. Specifically, the mechanical work on the ply (or the sheet) is accompanied by a loosening of the interfibre links of the embossed zones.
In the case of a multi-ply sheet, the embossing may be carried out individually on each ply and then the already embossed plies may be combined thanks to a marrying cylinder. Application WO 2004/065113 illustrates an example of this type of combining operation.
However, such a marrying cylinder is complex to produce especially when all its external surface must be covered with a strip of hard material rolled in a helix.
In one or other method of producing a multi-ply sheet, the two (or even more) plies are embossed and then combined by passing the sheet thus treated and formed between an engraved cylinder and a marrying cylinder.
The combining operation may pose problems particularly of wear of the engraved cylinder and/or of the marrying cylinder.
The wear is accentuated when high pressures and/or speeds are necessary.
A first known approach consists in covering the external surface of the marrying cylinder for example with a shell.
Application FR 2 801 833 discloses a marrying cylinder (for example) onto which a sleeve is mounted, a layer called an attachment layer being interposed between the cylinder and the sleeve. The attachment layer may be considered to be an “elastic” sub-layer that absorbs the pressure variations and also the manufacturing differences of each of the cylinders.
However, in use, it was revealed that the manufacturing differences and the pressure variations absorbed by this type of cylinder are insufficient. Premature and intermittent wear appeared, particularly if the cylinders operate at high speeds, from approximately 300 m/min.
In addition, the pressure on the sheet at the passage (or nip) between the cylinders accentuates the wear thereof; the external layer is damaged in places.
Naturally, all these deficiencies have negative consequences on the sheets formed which, for example, are not sufficiently combined (they delaminate); the result therefore is a production of uneven, or even generally bad, quality.
This is acceptable neither for the manufacturer nor for the user.
There is therefore a need to combine plies made of tissue paper in a manner that is reliable, simple, without bonding and that obviates the problems specified hereinabove.